when come to multi database files, i come across lots of crash

[sigusr1]

When I looking for an embeded key value database, I was impressed by your implementation method, so i decide to use it.

However, when come to multi database files, i come across lots of crash!!!

You can refer to my modification.

https://github.com/sigusr1/lightkv/commit/5a8ed1577f40b96f70a8b7022f1e427ae0128708